The only thing you miss by not going to SD in the summer is Wall Drug, which to me is a huge attraction. It's cheesy as heck, but it is so much fun. Other than that October would probably be the perfect time to visit. If you're like me, and hate heat, don't do the Badlands during July or August. Well, unless you like to see your skin melting off like a bad horror flick while you walk through the sand. In October though, it'll will start cooling off at night, and the days will be bearable.
Allison - You shoulda gone to the 1880s town. The very first building you to go is the complete memorabelia collection of Dances With Wolves. The props, the stills, the costumes. The horse that Kevin Costner rode was there too until he passed a few years back. I got to see him and pet him, and he was such a sweetheart. I was very sad when he passed. Dances With Wolves was filmed in SD, so it was quite a big deal for the museum to be there.
Fun trivia - A part of the movie Twister was also filmed here. They wanted to film in the sunflower fields that are over by Aberdeen. Massive amounts of acreage of nothing but sunflowers. It is absolutely gorgeous in season. They had everything set up for filming the next day, cameras ready, props out, the works and what happened? Yup, a tornado came through and wiped the entire thing out. They had to bring in all new cameras, booms, everything. I found that mildly amusing.
OH! I should also mention Wounded Knee is here. The actual place it happened is on the Rosebud Reservation. The museum though just outside of Wall. The museum is.....I don't even know how to describe it. It's nothing fancy. It's wall after wall of reading and pictures. It is so incredibly sobering. You can't help but leave with your heart just about torn out. I couldn't leave without buying three or four books either. You just struggle to understand how and why this could have happened.
Another place is Custer's last Stand. It's just fields, but the diagrams are there, the monuments. You can walk the entire place and read who died where and how many. A little background - Custer was an idiot. His informants told him that there were 10,000 Native Americans gathered there for a peaceful get together, and for some bizarre reason he thought he could "take them out." So he charged them with something like 500 men? It was absolutely a massacre. The natives weren't even expecting it, but you can bet they took advantage of it. Strangely enough, even though he had plagued their people, they considered Custer to be a leader and therefore deserving of honor, so while they hacked everybody else to bits and scalped them (including Custer's brother), they left "Yellow Hair" completely intact, laying in state surrounded by his dead soldeirs.
Soooo much stuff to do in SD. Did you know that Laura Ingalls Wilder's homestead is here? Where she grew up with Ma, Pa, and the fam? You can tour the homestead, wear period costumes, all kinds of stuff.
Okay, enough. I need to go to work now.